1. Field of the Invention
The present invention generally pertains to the art of sensors for measuring magnetic fields. More particularly, this invention relates to a sensor for measuring both static and oscillating magnetic fields.
2. Discussion of the Prior Art
Typically, magnetic sensors are divided into two categories. The first category includes those sensors that are designed to measure static magnetic fields. The second category includes those sensors designed to measure oscillating magnetic fields. For example, the Earth's main magnetic field is quasi-static, while those magnetic fields produced by alternating current electricity are oscillating fields. Sensors designed to measure static or low frequency fields have an upper frequency response generally around a few thousand hertz. For example, fluxgate and optically pumped magnetometers fall into this category. The oscillating magnetic field sensor category is dominated by magnetic induction sensors, which typically operate from 10 Hz to 100 kHz or higher. The exceptions to this rule include the superconducting quantum interface devices (SQUID). SQUID sensors can measure from a static field to a field oscillating at approximately 1 MHz. Magnetoresistive sensors can also measure static and oscillating magnetic fields. However, SQUIDS are difficult to use in practical applications and magnetoresistive sensors lack sensitivity adequate for many applications.
There are a number of emerging applications that require magnetic measurements from a static field to one oscillating in the order of 20 kHz, for which the overall system size and weight are important criteria. For these systems, using separate static and oscillating magnetic sensors is unfavorable. For example, future advanced detection systems for unexploded ordnance will require a combination of static and oscillating magnetic measurements, preferably three-axis vector signals, to provide characterization of target shape and reduce false alarm rate. Present oscillating magnetic field sensors used for unexploded ordnance detection are predominately based on using induction coils. To measure the static magnetic field, a second sensor, usually an optically pumped magnetometer or a fluxgate magnetometer, is required. The need for independent alternating static and magnetic sensors increases system size, weight, and cost, while preventing a rigorously co-located measurement of the target response. Another example of sensors which need a wide frequency response range is sensors used for atmospheric and planetary magnetic fields. The overall size and weight of such sensors are critical factors. In these fields, the largest possible upper operating frequency is typically desired. A further example is sensors used as part of electromagnetic surveillance systems.
In all of these examples, measurements of multiple components of the magnetic field are generally desired. Further, there is a concern that when separate sensors are used, i.e., one sensor for static fields and another sensor for oscillating magnetic fields, the metallic or magnetically permeable components of one magnetic sensor will disturb the field measured by the other sensor. In particular, a static magnetometer, such as a fluxgate magnetometer, cannot generally be put in close proximity to an induction sensor that uses a high permeability core because the signal detected by the fluxgate magnetometer will be affected by the distortion of the magnetic field caused by the induction sensor core.
Accordingly, there exists a need in the art for a compact magnetic sensor system capable of sensing magnetic fields that oscillate from a frequency of zero to 100 kHz and higher. Further, since using separate static and oscillating magnetic sensors is not favorable, a single compact sensor with a capability to operate in the entire frequency range is desired.